Step stool



FIG. 1 is a perspective view of a step stool showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a right side view thereof, a left side view being a mirror image of the right side view;

FIG. 5 is a top view thereof; and,

FIG. 6 is a bottom view thereof.

The broken lines shown in the drawings depict portions of the step stool that form no part of the claimed design. 

CLAIM The ornamental design for a step stool, as shown and described. 